Genetic and environmental variation in heartwood colour of Australian blackwood (Acacia melanoxylon R.Br.)

Australian blackwood (Acacia melanoxylon R.Br.) is a high-quality appearance-grade timber species native to eastern Australia. It is characterised by dark-coloured heartwood, with colour varying from pale straw to red-brown and walnut brown, and pale cream-coloured sapwood. This wide range in heartwood colour is expressed as between- and within-tree variation, and is regarded as a problem in markets where colour consistency is important. To understand the genetic and environmental control of heartwood colour, multiple colour measurements were taken from stem cores from 16 open-pollinated family collections of blackwood planted across three 19-year-old progeny trials. Data was analysed with a mixed model with within-tree variation modelled using a three-node cubic spline. Significant genetic, environmental and genetic-by-environment interactions were detected, not only in the variation in between-tree mean heartwood colour but also in the pattern of within-tree heartwood colour variation. In general, heartwood colour became darker, less yellow and more red towards the sapwood/heartwood boundary. To control the between- and within-tree heartwood colour variation in blackwood plantations, care is needed to not only select genotypes with the desired wood colour and colour variation but also site characteristics that allow expression of that colour.